Projection television lens systems having improved modulation transfer functions

ABSTRACT

Six-component projection lens systems (13) for use in CRT projection televisions (10) are provided. To provide an improved overall modulation transfer function across the field of view of the lens, the system&#39;s first lens element (L1) has an object side surface (S2) which has a best fit spherical surface which is convex to the CRT (16). To provide partial axial color correction, the system&#39;s second lens element (L2) is made of a high dispersion material, such as styrene.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International Application No. PCT/US99/13795 filed Jun. 18, 1999, and claims the benefit under 35 USC §119(e) of U.S. Provisional Application No. 60/090,518 filed Jun. 24, 1998, the contents of both of which in their entireties are hereby incorporated by reference.

International Application No. PCT/US99/13795 was published in English as WO 99/67662 on Dec. 29, 1999.

FIELD OF THE INVENTION

This invention relates to projection lens systems for use in projection televisions and, in particular, to projection lens systems having improved correction of higher order off-axis aberrations, especially coma and astigmatism. and thus improved overall modulation transfer functions across the field of view of the lens in comparison to prior lens systems having a similar construction.

BACKGROUND OF THE INVENTION

U.S. application Ser. No. 08/715,028, now U.S. Pat. No. 5,808,804, the contents of which were published on Mar. 26, 1997 as EPO Patent Publication No. 764,865, discloses projection lens systems for CRT projection televisions which employ five lens elements and a negative lens unit which is associated with the CRT during use of the lens system.

Although the lens systems of U.S. Pat. No. 5,808,804 work successfully for many applications, it has been found that the level of correction of higher order off-axis aberrations, especially coma and astigmatism, achieved by these prior lens systems result in less than desired overall modulation transfer functions (MTFs) across the field of view of the lens.

The present invention addresses this observed property of these prior lenses while maintaining their otherwise desirable characteristics.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide improved projection lens systems for use in projection televisions and, in particular, in rear projection televisions.

More particularly, it is an object of the invention to provide projection lens systems capable of covering a semi-field of view of up to about 40° at a f-number at infinity on the order of 1.0, while at the same time providing an image quality which is sufficiently high to satisfy the requirements of modern consumer projection TV systems, including the requirements of high definition televisions (HDTV).

It is an additional object of the invention to provide projection lens systems having the foregoing characteristics which are also partially corrected for axial color.

It is a further object of the invention to provide projection lens systems which can be manufactured at a reasonable cost.

To achieve the foregoing and other objects, the invention provides a projection lens system for use in combination with a cathode ray tube comprising in order from the image (screen) side:

(a) a first lens element (L1) which has a positive power, an image side surface (S₁), and an object side surface (S₂), wherein:

(i) at least one of the image and object side surfaces is aspherical; and

(ii) the object side surface has a best fit spherical surface which is convex to the cathode ray tube, i.e., the object side surface has a best fit spherical radius R_(2BF) which, under the standard convention for assigning radii to optical surfaces, is less than zero (see Table 5);

(b) a second lens element (L2) which has a negative power, at least one aspheric surface, and is composed of a high dispersion material (e.g., styrene);

(c) a third lens element (L3) which has a positive power and provides a substantial portion of the power of the lens system;

(d) a fourth lens element (L4) which has a weak power, at least one aspheric surface, and an overall meniscus shape which is concave to the cathode ray tube;

(e) a fifth lens element (L5) which has a positive power and at least one aspheric surface; and

(f) a lens unit (U_(N)) which has a strong negative power, is associated with the CRT during use of the lens system, and provides most of the correction for the field curvature of the lens system.

In certain preferred embodiments, the first element's image side surface (S₁) has a clear aperture CA₁ which satisfies the following relationship:

½CA ₁ >Y ₁  (1)

where Y₁, is the height of the axial marginal ray at S₁. By means of this limitation, more effective correction of off-axis aberrations can be achieved using the portion of the first lens element which lies between Y₁ and the clear aperture of the lens. Preferably, the first element's image side surface satisfies the following relationship:

½CA ₁>1.1·Y ₁.  (2)

In other preferred embodiments, the first element's image side surface has a best fit radius R_(1BF) which satisfies the following relationship:

|R _(1BF) |>|R _(2BF)|.  (3)

That is, the best fit power of the CRT side of the first lens element is greater than the best fit power of the screen side of that element.

In still further preferred embodiments, the lens system has some or all of the following characteristics: (1) each of the first, second, fourth, and fifth lens elements has two aspheric surfaces; (2) the third lens element is biconvex and provides the majority of the power of the lens system, i.e., the focal length of the third lens element is between about 0.8 and about 1.6 times the focal length of the entire lens system; and (3) the fourth lens element has a positive power.

As used herein, the term “weak” is used to describe an element whose focal length has a magnitude which is at least about 2.5 times the effective focal length of the entire lens system, and the term “strong” is used to describe an element or unit whose focal length has a magnitude which is less than about 2.5 times the effective focal length of the entire lens system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 2A, 3A, and 4A are schematic side views of lens systems constructed in accordance with the invention.

FIGS. 1B, 1C, 2B, 2C, 3B, 3C, and 4B are monochromatic MTF/OTF plots for the lens systems of FIGS. 1A, 2A, 3A, and 4A, respectively.

FIG. 5 is a monochromatic MTF/OTF plot. for the lens system of FIG. 3 of U.S. Pat. No. 5,808,804.

FIG. 6 is a schematic diagram of a projection TV employing a lens system constructed in accordance with the invention.

Table 6 sets forth the system parameters for the MTF/OTF plots of FIGS. 1B, 1C, 2B, 2C, 3B, 3C, 4B, and 5.

The foregoing drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As discussed above the present invention is directed to providing improved projection lenses of the general type disclosed in U.S. Pat. No. 5,808,804, the contents of which are incorporated herein by reference.

In particular, the lenses of the invention have a first lens element which has an object side surface which is convex, rather than concave, to the short conjugate (object) side of the lens system. In addition, to allow for improved correction of off-axis aberrations, the long conjugate (image) side surface of the first lens element has a clear aperture which is substantially greater than the height of the axial marginal ray at that surface. Although such an increase in clear aperture size and thus increase in element size is in general undesirable, in accordance with the invention, it has been found that the increase in size is well justified by the resulting improvement in overall MTF across the field of view of the lens.

The second lens element of the lens systems of the invention has a negative power and is composed of a high dispersion material so as to provide the system with at least some axial color correction. In particular, the negative second lens element is composed of a material having a higher dispersion than the dispersion of the material used for the positive third lens element.

As used herein, a high dispersion material is one having a dispersion like that of flint glass. More particularly, a high dispersion material is one having a V-value ranging from 20 to 50 for an index of refraction in the. range from 1.85 to 1.5, respectively. In contrast, a low dispersion material is one having a dispersion like that of crown glass or, in terms of V-values, one having a V-value ranging from 35 to 75 for an index of refraction in the range from 1.85 to 1.5, respectively.

A preferred high dispersion material for use in constructing the negative second lens element is styrene and a preferred low dispersion material for use in constructing the positive third lens element is a crown-type glass. In place of styrene, other plastics having flint-like dispersions can be used to prepare the negative second lens element, including polycarbonates and copolymers of polystyrene and acrylic such as NAS. See The Handbook of Plastic Optics, U.S. Precision Lens, Inc., Cincinnati, Ohio, 1983, pages 17-29.

Each of elements 1, 2, 4, and 5, as well as the field flattener lens unit (U_(N)), has at least one aspherical surface to facilitate the correction of the aberrations of the lens system and, preferably, each of the lens elements has two aspheric surfaces. Because the lens surfaces are aspheric, the overall shapes of the elements are preferably described in terms of best fit spherical surfaces, rather than the radii of curvature of the elements at the optical axis, although in many cases the description of the overall shapes of the elements in terms of best fit spherical surfaces on the one hand and in terms of the radii of curvature at the optical axis on the other will be the same. See Dunham, C. B., and C. R. Crawford, “Minimax Approximation by a Semi-Circle,” Society for Industrial and Applied Mathematics, Vol. 17, No. Feb. 1, 1980.

To maintain a relatively low cost of the lens system, all the aspherical elements are designed to be manufactured in optical plastic. In particular, for ease of molding, a fairly uniform thickness is maintained across the aperture of each of these elements. Because they are plastic, it is desirable to keep the power of the elements as low as possible so as to minimize shifts in focus of the lens system with changes in ambient temperature. As discussed above, the power element (element 3) is preferably made out of glass.

FIGS. 1-4 illustrate various projection lenses constructed in accordance with the invention. Corresponding prescriptions appear in Tables 1-4. HOYA or SCHOTT designations are used for the glasses employed in the lens systems. Equivalent glasses made by other manufacturers can be used in the practice of the invention. Industry acceptable materials are used for the plastic elements.

The aspheric coefficients set forth in the tables are for use in the following equation: $z = {\frac{{cy}^{2}}{1 + \left\lbrack {1 - {\left( {1 + k} \right)c^{2}y^{2}}} \right\rbrack^{1/2}} + {Dy}^{4} + {Ey}^{6} + {Fy}^{8} + {Gy}^{10} + {Hy}^{12} + {Iy}^{14}}$

where z is the surface sag at a distance y from the optical axis of the system, c is the curvature of the lens at the optical axis, and k is a conic constant, which is zero except where indicated in the prescriptions of Tables 1-4.

The designation “a” associated with various surfaces in the tables represents an aspheric surface, i.e., a surface for which at least one of D, E, F, G, H, or I in the above equation is not zero. The designation “c” represents a conic surface, i.e., a surface for which k in the above equation is not zero. The tables are constructed on the assumption that light travels from left to right in the figures. In actual practice, the viewing screen will be on the left and the CRT will be on the right, and light will travel from right to left.

The CRT faceplate constitutes surfaces 15-16 in Tables 1, 2, and 4, and surfaces 13-14 in Table 3. A coupling fluid is located between surfaces 14-15 in Tables 1, 2, and 4, and surfaces 12-13 in Table 3. Other than for the CRT faceplate of Table 4 which is composed of FDS9.glass, the material designations for the coupling fluid and the faceplate are set forth as six digit numbers in the tables, where a N_(e) value for the material is obtained by adding 1,000 to the first three digits of the designation, and a V_(e) value is obtained from the last three digits by placing a decimal point before the last digit. Optional vignetting surfaces are shown at surfaces 3 and 10 in Tables 1, 2, and 4.

All dimensions given in the prescription tables, as well as in Tables 5 and 6, are in millimeters.

Table 5 summarizes the ½CA₁, Y₁, R_(1BF), and R_(2BF) values for the projection lens systems of Tables 1-4. As shown in this table, the lens systems of the examples satisfy requirements (1)-(3) set forth above. Indeed, for each of these examples, the ratio of ½CA₁ to Y₁ is at least equal to 1.3.

For comparison, the ½CA₁, Y₁, R_(1BF), and R_(2BF) values for Example 3 of U.S. Pat. No. 5,808,804, the only example of that patent which has a high dispersion second lens element, are 40.095 mm, 40.095 mm, 164.3 mm, and 188.1 mm, respectively. Requirements (1)-(3) are thus not met by this prior example. (Examples 1 and 4 of U.S. Pat. No. 5,808,804 have |R_(1BF)|>|R_(2BF)|, but like all of the other examples of that patent, the R_(2BF) values for these examples are not less than zero, i.e., the object side surface of the first lens element is not convex towards the CRT.)

FIGS. 1B, 1C, 2B, 2C, 3B, 3C, and 4B are geometric MTF plots for the lenses of Examples 1-4. In particular, these plots show the through-focus MTF on the left and the optical transfer function (OTF) at best axial focus on the right. The data are shown for five field points, viz., the axis, 0.35H, 0.70H, 0.85H and 1.0H, where H is the maximum field height on the screen. The actual field heights are shown for the right hand plots. These field heights apply to both the right hand and left hand plots and are in millimeters.

The through-focus data are at the indicated spatial frequency in cycles per millimeter, i.e., at 2 cycles/mm except for the on-axis plot which is at 2 and 5 cycles/mm. A spatial frequency of 2 cycles/mm is often used in evaluating the performance of projection television systems since it. corresponds to approximately 400 TV lines in the horizontal direction for a five-inch diagonal image on a typical seven-inch CRT tube. A 400. TV line resolution corresponds to the maximum resolution specification for most laser disc players.

Both the through-focus and best-focus data indicate tangential (solid curves) and sagittal (dashed curves) MTF. The modulus scale is on the left of each block and runs from zero to one. The phase of the OTF is shown as a dotted curve in the best-focus plots. The scale for the phase is indicated on the right of each best-focus block and is in radian measure. All the OTF data are for a wavelength of 546.1 nanometers. The axial focus shifts for the best focus plots of FIGS. 1B, 1C, 2B, 2C, 3B, 3C, 4B, and 5 are 0.02, −0.004, 0.014, −0.018, 0.005, −0.055, −0.006, and −0.077, respectively. These focus shifts are relative to the zero position of the through-focus plots. The best-focus plane is at the peak of the axial through-focus plot.

FIG. 5 shows a corresponding MTF plot for Example 3 of U.S. Pat. No. 5,808,804. (The plot of FIG. 5 differs from the plot of FIG. 3B of U.S. Pat. No. 5,808,804 in that a diffractive MTF is shown in FIG. 3B, while a geometric MTF is shown in FIG. 5.) An examination of FIG. 5 and FIGS. 1B, 1C, 2B, 2C, 3B, 3C, and 4B shows that the projection lenses of the invention achieve improved overall MTFs across the field of the lens in comparison with the MTFs achieved with the construction of U.S. Pat. No. 5,808,804.

FIG. 6 is a schematic diagram of a CRT projection television 10 constructed in accordance with the invention. As shown in this figure, projection television 10 includes cabinet 12 having projection screen 14 along its front face and slanted mirror 18 along its back face. Module 13 schematically illustrates a lens system constructed in accordance with the invention and module 16 illustrates its associated CRT tube. In practice, three lens systems 13 and three CRT tubes 16 are used to project red, green, and blue images onto screen 14.

Although specific embodiments of the invention have been described and illustrated, it is to be understood that a variety of modifications which do not depart from the scope and spirit of the invention will be evident to persons of ordinary skill in the art from the foregoing disclosure.

TABLE 1 Surf. Clear Aperture No. Type Radius Thickness Glass Diameter  1 a 252.8463 12.00000 ACRYLIC 120.54  2 ac −409.4738  9.99443 117.38  3 ∞  5.89278  94.00  4 a 626.2584  8.80000 STYRENE  93.03  5 a 90.3502  2.10000  96.62  6 136.2313 23.00000 BACD5  98.75  7 −136.2313  1.00000  99.21  8 a 73.4121 13.00000 ACRYLIC  93.20  9 a 122.0014  6.65119  92.17 10 ∞  5.90000  87.35 11 a 1027.6169 10.50000 ACRYLIC  93.64 12 a −159.0282 Space 1  93.55 13 a −53.0869  5.30000 ACRYLIC  98.24 14 −56.0000 12.00000 430500 104.00 15 ∞ 14.80000 565500 175.00 16 −600.0000 Image distance 190.00 Symbol Description a—Polynomial asphere c—Conic section Object and Image Surface Surface Radius Image −599.9999 Conics Surface Number Constant 2 −1.9398E + 02 Even Polynomial Aspheres Surf. No. D E F G H I  1 −5.5195E − 07 −1.0713E − 10 3.1479E − 14 −5.7484E − 18 6.1604E − 22 −8.9173E − 26  2 1.1325E − 07 −7.2072E − 11 −8.3839E − 15 −1.2759E − 20 −8.5801E − 22 1.6785E − 25  4 −2.8412E − 07 −3.4636E − 11 −1.0778E − 13 3.5508E − 17 −8.3433E − 21 1.5423E − 24  5 −1.4043E − 06 1.8900E − 10 2.8916E − 15 −3.9504E − 17 1.8623E − 20 −1.9552E − 24  8 −1.3276E − 06 2.2532E − 10 −3.7469E − 13 6.8848E − 17 6.1456E − 20 −2.0995E − 23  9 −1.1185E − 06 −5.9118E − 10 −3.0478E − 14 7.5880E − 17 1.5753E − 20 −1.0854E − 23 11 9.1156E − 07 1.7192E − 10 7.0610E − 13 −5.3712E − 16 1.8891E − 19 −3.5346E − 23 12 1.1530E − 06 1.5285E − 09 −8.8458E − 13 9.0388E − 16 −4.2610E − 19 5.8397E − 23 13 −1.7092E − 06 2.6669E − 09 −3.0629E − 12 2.0327E − 15 −6.9802E − 19 9.9679E − 23 Variable Spaces Focus Space 1 Focal Image Pos. T(12) Shift Distance 1 45.530 0.340   0.009 2 46.724 0.037 −0.001 First-Order Data f/number 1.23 1.25 Magnification −0.0931 −0.1112 Object Height −1008.0 −851.00 Object Distance −1209.6 −1021.6 Effective Focal Length 102.31 101.37 Image Distance 0.86727E − 02 −.11816E − 02 Overall Length 1386.0 1199.3 Forward Vertex Distance 176.48 177.66 Barrel Length 176.47 177.66 Stop Surface Number 9 9 Distance to Stop 0.00 0.00 Stop Diameter 92.490 92.395 Entrance Pupil Distance 66.139 66.139 Exit Pupil Distance −69.077 −69.648 First Order Properties of Elements Element Number Surface Numbers Power f 1  1  2 0.31398E − 02 318.49 2  4  5 −0.56004E − 02 −178.56 3  6  7 0.84102E − 02 118.90 4  8  9 0.29157E − 02 342.97 5 11 12 0.35750E − 02 279.72 6 13 14 −0.19286E − 03 −5185.0 7 14 15 −0.76786E − 02 −130.23 8 15 16 0.94167E − 03 1061.9 First-Order Properties of Negative Unit when Associated with CRT Element Numbers Surface Numbers Power f 6 8 13 16 −0.70155E − 02 −142.54

TABLE 2 Surf. Clear Aperture No. Type Radius Thickness Glass Diameter  1 a 335.2195 11.00000 ACRYLIC 109.16  2 c −281.5686 13.01901 104.02  3 ∞  2.59344  84.83  4 a 253.0301  8.00000 STYRENE  82.28  5 a 68.2415  1.55607  82.55  6 97.9238 23.5000 SK5  83.60  7 −97.9238  0.43224  84.02  8 a 68.7895 10.00000 ACRYLIC  78.12  9 a 88.5713  7.32837  76.71 10 ∞  6.91585  73.10 11 a −4092.0380  9.00000 ACRYLIC  74.80 12 a −128.8796 Space 1  74.11 13 a −53.3113  4.00000 ACRYLIC  84.07 14 −55.0000  9.00000 437500  88.80 15 ∞ 14.10000 563500 130.00 16 −350.0000 Image distance 140.00 Symbol Description a—Polynomial asphere c—Conic section Object and Image Surface Surface Radius Image −350.0000 Conics Surface Number Constant 2 −5.5055E + 00 Even Polynomial Aspheres Surf. No. D E F G H I  1 −1.1613E − 06 4.5735E − 11 7.0597E − 14 −2.6441E − 17 4.8355E − 21 −4.1038E − 25  4 −5.9817E − 07 3.0494E − 11 −4.0537E − 13 1.4208E − 16 1.2703E − 20 −7.4163E − 24  5 −1.6454E − 06 1.0131E − 10 3.0313E − 14 −1.7207E − 16 1.4227E − 19 −2.9171E − 23  8 −2.3665E − 06 3.8275E − 10 −8.2617E − 13 1.3440E − 16 1.9749E − 19 −4.7339E − 23  9 −2.5734E − 06 −1.0864E − 09 1.9629E − 13 4.9055E − 17 7.0842E − 20 −1.9655E − 23 11 1.3602E − 06 7.6108E − 10 1.4933E − 12 −1.4939E − 15 6.8027E − 19 −1.7042E − 22 12 2.0592E − 06 2.4475E − 09 −1.5705E − 12 2.7816E − 15 −1.7939E − 18 3.1821E − 22 13 −3.5626E − 06 7.6203E − 09 −9.8331E − 12 6.9844E − 15 −2.5972E − 18 4.0852E − 22 Variable Spaces Focus Space 1 Image Focal Pos. T(12) Distance Shift 1 37.414 0.028 0.515 2 37.819 0.015 0.379 First-Order Data f/number 1.23 1.24 Magnification −0.0934 −0.1005 Object Height −762.00 −698.50 Object Distance −1036.1 −966.29 Effective Focal Length 89.093 88.843 Image Distance 0.28374E − 01 0.15126E − 01 Overall Length 1194.0 1124.6 Forward Vertex Distance 157.89 158.28 Barrel Length 157.86 158.26 Stop Surface Number 6 6 Distance to Stop 33.26 33.26 Stop Diameter 85.534 85.690 Entrance Pupil Distance 58.716 58.716 Exit Pupil Distance −64.950 −65.170 First Order Properties of Elements Element Number Surface Numbers Power f 1  1  2 0.32076E − 02 311.76 2  4  5 −0.62642E − 02 −159.64 3  6  7 0.11541E − 01 86.651 4  8  9 0.18711E − 02 534.46 5 11 12 0.37134E − 02 269.29 6 13 14 −0.61709E − 04 −16205. 7 14 15 −0.79527E − 02 −125.74 8 15 16 0.16086E − 02 621.67 First-Order Properties of Negative Unit when Associated with CRT Element Numbers Surface Numbers Power f 6 8 13 16 −0.63613E − 02 −157.20

TABLE 3 Surf. Clear Aperture No. Type Radius Thickness Glass Diameter  1 a 183.8607 10.00000 ACRYLIC  91.80  2 c −334.6885  8.91596  85.67  3 a −404.0650  7.00000 STYRENE  78.10  4 a 109.3349  1.30000  77.20  5 99.3167 21.00000 SK18  79.16  6 −99.3167  0.50000  79.19  7 a 59.7183  9.00000 ACRYLIC  71.12  8 a 77.4899 12.55810  69.53  9 a 429.2780  9.00000 ACRYLIC  68.06 10 a −123.5628 Space 1  67.84 11 a −40.0420  4.00000 ACRYLIC  71.92 12 −45.0000  9.00000 432500  77.25 13 ∞ 14.10000 562500 130.00 14 −350.0000 Image distance 130.00 Symbol Description a—Polynomial asphere c—Conic section Object and Image Surface Surface Radius Image −350.0000 Conics Surface Number Constant 2 −2.1039E + 02 Even Polynomial Aspheres Surf. No. D E F G H I  1 −1.5279E − 06 −2.5091E − 10 3.1049E − 13 −5.1932E − 17 3.9878E − 22 −3.2253E − 25  3 −2.4488E − 07 −2.6319E − 10 −8.7145E − 13 4.8158E − 16 −3.5438E − 20 −6.9851E − 24  4 −2.4369E − 06 4.0362E − 10 1.3770E − 13 −4.0433E − 16 3.7989E − 19 −9.0044E − 23  7 −4.2376E − 06 7.3887E − 10 −1.9975E − 12 6.4526E − 16 9.6290E − 19 −5.5629E − 22  8 −4.0474E − 06 −1.6877E − 09 7.4813E − 15 7.4285E − 16 2.3295E − 19 −3.2497E − 22  9 2.2387E − 07 −2.4544E − 10 3.8443E − 12 −4.9023E − 15 4.3072E − 18 −1.6390E − 21 10 9.7330E − 07 4.9031E − 09 −7.6044E − 12 1.0596E − 14 −5.7514E − 18 8.8474E − 22 11 −4.5889E − 06 1.6253E − 08 −3.1557E − 11 3.6166E − 14 −2.2102E − 17 5.6074E − 21 Variable Spaces Focus Space 1 Focal Image Pos. T(10) Shift Distance 1 28.031   0.191 0.000 2 28.567 −0.039 0.000 First-Order Data f/number 1.20 1.21 Magnification −0.0855 −0.0980 Object Height −800.00 −698.50 Object Distance −982.20 −862.69 Effective Focal Length 77.294 76.872 Image Distance 0.00 0.00 Overall Length 1116.6 997.63 Forward Vertex Distance 134.41 134.94 Barrel Length 134.41 134.94 Stop Surface Number 5 5 Distance to Stop 31.59 31.59 Stop Diameter 79.326 79.016 Entrance Pupil Distance 46.660 46.660 Exit Pupil Distance −54.522 −54.792 First Order Properties of Elements Element Number Surface Numbers Power f 1  1  2 0.41344E − 02 241.87 2  3  4 −0.69491E − 02 −143.90 3  5  6 0.12380E − 01 80.773 4  7  8 0.22137E − 02 451.73 5  9 10 0.51187E − 02 195.36 6 11 12 −0.99632E − 03 −1003.7 7 12 13 −0.96000E − 02 −104.17 8 13 14 0.16057E − 02 622.78 First-Order Properties of Negative Unit when Associated with CRT Element Numbers Surface Numbers Power f 6 8 11 14 −0.89863E − 02 −111.28

TABLE 4 Surf. Clear Aperture No. Type Radius Thickness Glass Diameter  1 a −2676.2499  8.00000 ACRYLIC 66.17  2 c −52.7129 12.04758 64.34  3 ∞  1.42639 49.32  4 a 560.8589  4.40000 STYRENE 49.27  5 a 34.1346  0.88000 50.91  6 56.0863 15.00000 SK18A 50.84  7 −56.0863  0.50000 51.14  8 a 32.7444  8.00000 ACRYLIC 49.09  9 a 40.8753  1.24197 48.17 10 ∞  2.00000 47.75 11 a −900.0738  6.00000 ACRYLIC 45.45 12 a −55.2870 16.97013 44.86 13 a −24.3908  3.00000 ACRYLIC 43.45 14 −27.0000 10.00000 432500 46.26 15 ∞  2.00000 FDS9 65.00 16 ∞ −0.00011 65.00 Symbol Description a—Polynomial asphere c—Conic section Conics Surface Number Constant 2 −6.1972E + 00 Even Polynomial Aspheres Surf. No. D E F G H I  1 −4.8632E − 06 1.2763E − 09 3.4380E − 12 −6.1097E − 15 4.2936E − 18 −1.1331E − 21  4 1.1839E − 06 −4.2905E − 09 −1.9417E − 11 2.8597E − 14 −6.6246E − 18 −3.4765E − 21  5 −1.2812E − 05 5.5247E − 09 2.6914E − 12 −4.3688E − 14 8.9033E − 17 −5.3343E − 20  8 −1.9297E − 05 1.2974E − 08 −6.1575E − 11 3.9989E − 14 1.6017E − 16 −1.6699E − 19  9 −1.8070E − 05 −2.0770E − 08 1.2949E − 11 1.8452E − 14 6.6601E − 17 −1.0037E − 19 11 9.8735E − 07 5.2566E − 09 9.4191E − 11 −3.0032E − 13 5.0136E − 16 −5.0250E − 19 12 4.1978E − 06 2.7997E − 08 −9.0330E − 11 5.7192E − 13 −1.3969E − 15 9.4989E − 19 13 −1.4926E − 05 8.6673E − 08 −4.6562E − 10 1.5639E − 12 −2.7407E − 15 2.0333E − 18 First-Order Data f/number 1.05 Magnification −0.0526 Object Height −610.00 Object Distance −993.533 Effective Focal Length 49.5523 Image Distance −.113249E − 03 Stop Surface Number 6 Focal Shift 0.51912 Overall Length 1085.00 Forward Vertex Distance 91.4660 Barrel Length 91.4661 Entrance Pupil Distance 25.6144 Exit Pupil Distance −34.9774 Stop Diameter 50.182 Distance to Stop 0.00 First Order Properties of Elements Element Number Surface Numbers Power f 1  1  2 0.91920E − 02 108.79 2  4  5 −0.16318E − 01 −61.283 3  6  7 0.21673E − 01 46.141 4  8  9 0.39753E − 02 251.56 5 11 12 0.84022E − 02 119.02 6 13 14 −0.12129E − 02 −824.50 7 14 15 −0.16000E − 01 −62.500

TABLE 5 Example CA₁ Y₁ R_(1BF) R_(2BF) 1 60.3 45.9 −696.5 −296.7 2 54.6 39.3 −544.5 −294.3 3 45.9 35.3 1671.0 −520.4 4 33.1 24.9 −147.0  −75.1

TABLE 6 PARAMETERS FOR MTF/OTF PLOTS Figure Focal Length Magnification F/Number Object Height Image Height FIG. 1B 101.37 −0.111 1.25 −851.00 88.57 FIG. 1C 102.31 −0.093 1.23 −1008.00 87.81 FIG. 2B 88.84 −0.101 1.24 −698.50 67.15 FIG. 2C 89.09 −0.093 1.23 −762.00 68.03 FIG. 3B 76.87 −0.098 1.21 −698.50 63.19 FIG. 3C 77.29 −0.086 1.20 −800.00 63.28 FIG. 4B 49.55 −0.053 1.05 −610.00 31.75 FIG. 5 71.79 −0.101 1.07 −635.00 63.48 

What is claimed is:
 1. A projection lens system for use with a cathode ray tube comprising in order from the image side: (a) a first lens element which has a positive power, an image side surface, and an object side surface, wherein: (i) at least one of the image and object side surfaces is aspherical; and (ii) the object side surface has a best fit spherical surface which is convex to the cathode ray tube; (b) a second lens element which has a negative power, at least one aspheric surface, and is composed of a high dispersion material; (c) a third lens element which has a positive power and provides a substantial portion of the power of the lens system; (d) a fourth lens element which has a weak power, at least one aspheric surface, and an overall meniscus shape which is concave to the cathode ray tube; (e) a fifth lens element which has a positive power and at least one aspheric surface; and (f) a lens unit which has a strong negative power, is associated with the CRT during use of the lens system, and provides most of the correction for the field curvature of the lens system.
 2. The projection lens system of claim 1 wherein the high dispersion material is styrene.
 3. The projection lens system of claim 1 wherein each of the first, second, fourth, and fifth lens elements has two aspheric surfaces.
 4. The projection lens system of claim 1 wherein the third lens element is biconvex.
 5. The projection lens system of claim 1 wherein the third lens element provides the majority of the power of the lens system.
 6. The projection lens system of claim 1 wherein the third lens element is composed of a crown-type glass.
 7. The projection lens system of claim 1 wherein the fourth lens element has a positive power.
 8. The projection lens system of claim 1 wherein the lens system has a half angle field of view in the direction of the image of about 40 degrees.
 9. The projection lens system of claim 1 wherein the f-number at infinity of the lens system is about 1.0.
 10. A projection television set comprising a cathode ray tube, a screen, and a projection lens system for projecting light from the cathode ray tube onto the screen to form an image, said projection lens system comprising the projection lens system of claim
 1. 11. A projection television set comprising three cathode ray tubes, a screen, and three projection lens systems, one projection lens system being associated with each of the cathode ray tubes for projecting light from that tube onto the screen to form an image, each projection lens system comprising the projection lens system of claim
 1. 